Micromechanical gyroscopic sensors are, after pressure sensors, currently the most frequently produced sensors in silicon micromechanics. Modern micromechanical technologies permit a decisive reduction in sizes and costs as well as integration with the electronics of a sensor. This has enabled a large number of new application fields to be opened up. These include the areas of the automobile industry, biomechanics, consumer goods industry, industrial applications, military applications and navigation. Airbag sensors, Electronic Stability Control (ESC), stabilization of video cameras, 3-D mouse devices and sensor systems for navigation devices are only a few of the applications under development. The current largest market for micromechanical rotational speed sensors is the automotive industry. These sensors can be used to measure acceleration and rotational movement of a vehicle in order to prevent the vehicle from tipping over or skidding in dangerous driving situations by braking individual wheels or by intervention in the vehicle suspension system. If, nevertheless, an accident occurs, it is possible for “intelligent” triggering of the front and side airbags to take place.
The measuring principle is based on Foucault's Pendulum which, while oscillating freely in a plane, carries out a precession movement when subjected to a rotational movement. For the measurement of the rotational speed (angular speed, also yaw rate) one may choose between an open-loop or closed-loop operating principle. In both cases it is necessary for the “pendulum”, i.e., the micromechanical gyroscopic sensor, to be excited with a primary oscillation of a constant amplitude and constant direction.
In open-loop operation, energy is transmitted from the oscillation amplitude of the primary oscillation to a secondary oscillation due to an external rotational movement. The ratio of the amplitudes of the primary and secondary oscillation is proportional to the rotational speed of the sensor. In closed-loop operation, the secondary oscillation is compensated for by using a closed control loop, and the actuating variable (i.e., the restoring forces) of the controller being necessary for this is a measure of the secondary oscillation. Compensating for the secondary oscillation allows the bandwidth of the rotational speed sensor to be increased significantly.
The primary oscillation is excited in micromechanical rotational speed sensors, which are typically in the form of two oscillating capacitor plates, using electrostatic forces, i.e., the electrodes (i.e., the “capacitor plates”) of the rotational speed sensor oscillate by applying an electrical voltage to them. The amplitude and the frequency of the primary oscillation can furthermore also be adjusted by control loops to specific values (for example, to a frequency range of mechanical resonance). The rotational speed sensor can therefore form an electrically excited mechanical resonator. This generally requires information about the current position of the resonator, in simplified terms the position of the “capacitor plates”. For this purpose, the capacitance of the sensor can be measured. This is known to be a measure of the distance between the plates in plate capacitors.
In order to generate the primary oscillation and the restoring forces, analogous control voltages have to be generated in a costly way. If, in order to excite the primary oscillation of the rotational speed sensor or to compensate for the secondary oscillation and to measure the position of the respective oscillator, the same “capacitor plates” are used, these analog control voltages must also be switched over using multiplexers, depending on whether the sensor is currently excited or whether a measuring process is taking place, with the multiplexing of the analog signals with the required precision being a problem whose solution entails a considerable degree of expenditure on circuitry. However, there is a general need for a circuit for operating gyroscopic sensors which require the smallest possible degree of expenditure on circuitry while providing the necessary precision.